This proposal outlines a 2-year plan for investigative research into the immunopathogenesis of BK virus (BKV) in renal transplant recipients. A major clinical barrier to renal transplant medicine is the occurrence of polyomavirus-associated nephropathy (PVAN) due to BKV, with significant occurrences of BK viremia (13%) and PVAN (8%) in the post-transplant period. Optimal immunosuppression to ensure graft survival must be balanced with infectious risk. Antibody levels are poor predictors of risk of disease or response to antiviral therapy and viral DNA PCR-based assays provide no direct information on the immune status. However, there are currently no cellular immune monitoring assays in clinical use for renal transplant recipients. To fill this gap, we will use four highly optimized polychromatic flow cytometry (PFC) assays designed to assess CD4 and CD8 T cell responses within maturational subsets: CEF peptide pool (HLA class I restricted epitopes for CD8 responses), Candida CASTA (CD4 activity), CMV pp65 & IE-1 peptide pools (primarily CD8) and BKV peptide pools (CD4, CD8). We will perform longitudinal immune monitoring in 120 patients following renal transplantation (weeks 0, 4, 12, 24, & 48). Because few such PFC assays report robust measurements of such antigen specific T cell responses, we will aim to objectively quantify CD4/CD8 T cell responses in the early (0-3 month) and late (6-12 month) post-transplant periods of immune reconstitution. By objectively and longitudinally quantifying antigen specific immune responses, we ultimately aim to fill gaps in scientific knowledge and clinical practice for the individual titration of immunosuppressive and chemoprophylaxis regimens to improve transplant clinical care and outcomes